Abstract
Cultured Ryukyu ayu Plecoglossus altivelis ryukyuensis was found to be infected with a microsporidian parasite on Amami Oshima Island in January 2009. The parasite formed numerous cysts on the visceral organs and skin of the diseased fish. High temperature seemed to suppress the development of the parasite; in 2009 and 2010, the disease occurred in the 0-year fish from April to August, and afterwards the prevalence decreased dramatically in September and October. These characteristics well resembled glugeosis in ayu P. altivelis altivelis in other areas of Japan. This parasite seems to be indigenous to Amami Oshima Island, because it was detected in the museum specimens of wild Ryukyu ayu caught in 1991. Average dimensions of spores from Ryukyu ayu were slightly shorter and wider than those of Glugea plecoglossi, but the ranges of the measurements were overlapped. From the above characteristics and molecular analysis of SSU rRNA, it was concluded that the present microsporidian was included genus Glugea, but the specific identity remained to be clarified.
